1. Field of the invention
This invention relates to tear open closures for containers of the kind which have a pouring aperture and a venting aperture in one end of the container, and more particularly but not exclusively to tear open closures of plastics material for fitting into can ends suitable for seaming to can bodies containing a carbonated beverage.
2. Description of the Prior Art
British Patent Specification No. 1,389,351 describes a can of this kind provided with an opening device moulded from a plastics material so as to have a resilient aperture or lip portion defining a pouring aperture and a smaller venting aperture aligned with the container openings, the lip portion being airtightly connected to the can end; a resilient removable portion connected to the lip portion by a hinge between adjacent ends of said portions, the removable portion carrying plugs on its undersurface which are attached in an airtight but rupturable manner to the edges of the apertures in the lip portion; and a pull tab normally located adjacent the hinge and connected to said removable portion by two legs which encircle the removable portion and are connected to its further or outer end, adjacent the periphery of the can end. In order to open the device the user lifts the pull tab and the legs transmit the force exerted on the pull tab to the outer end of the removable portion so as to rupture the plugs from the pouring aperture and the vent aperture in turn. Continued pulling thus opens the pouring aperture and then the vent aperture and the hinge allows the removable portion to be extended away from the apertures to facilitate drinking or pouring, while keeping it attached ready for reclosure by fitting the plugs on the removable portion into the apertures.
The closure disclosed in British Patent Specification No. 1,389,351 thus consists of the aperture or lip portion, which includes two annular collar portions fitting tightly in the apertures, and, above the aperture portion, the removable portion which is connected to the aperture portion by the joints between the plugs and the collars and by the hinge.
The arrangement of the upper removable portion, the lower or aperture portion and the plug to collar joints gives rise to a plastics moulding which has rectilinear rigidity, like that of a box section, so that the collar portions are unable to tilt readily in relation to one another, as is necessary when the closure is to be fitted into a domed can end or when the can end is liable to assume a domed shape under internal pressure in the can. In such cases it is essential that the closure shall bend so that the axis of one collar is inclined to the axis of the other collar, so enabling each collar to fit correctly within the respective aperture in the domed can end panel. The rigid arrangement also means that the collar portions are unable to yield to accommodate any dimensional difference between the distance between the centres of the apertures in the can end and the distance between the centres of the collar portions. Whilst the apertures in the sheet metal of the can end are dimensionally stable, it is in the nature of plastics materials to exhibit some variation in shrinkage during setting after moulding, so that it is desirable to provide some degree of lateral float or flexibility for the collar portions with respect to one another in order to facilitate fitting of the collars into the can end.
Plastics materials as used for the closure have a degree of inherent flexibility, but it is not practicable to use a very flexible material because the collar portions would then tend to slip out of the apertures. Experience has shown that, when the closures are fitted into domed can ends or can ends which assume a domed shape under internal pressure, or when dimensional variations occur in the closures, the rigidity conferred by the arrangement referred to above results in stresses on the collar portions and consequent distortion giving rise to leakage.